1. Field of the Invention
The invention pertains broadly to laser gyroscope systems. More particularly, the invention relates to four-frequency laser gyroscope systems in which high precision output signals are required.
2. Description of the Prior Art
One of the primary problems which must be overcome to provide a useful laser gyroscope system is that of mode locking. In an uncompensated system in which no provision is made for overcoming this problem, for low angular rates of rotation, the frequency difference produced between wave pairs circulating in their laser gyroscope cavity are less than would be predicted were not for the mode locking phenomena. In fact, the actual frequency difference output only asymptotically approaches the desired linear relationship between output frequency difference and rate of rotation as the actual rate of rotation is increased.
Numerous laser gyroscope structures have been proposed for eliminating or substantially reducing this mode locking problem. Among the most successful of these systems is that shown and described in the U.S. Pat. Nos. 3,741,657 and 3,854,819, both to Keimpe Andringa and assigned to the present assignee, the specifications thereof being incorporated by reference herein. In the patented systems, beams of four seperate frequencies propagate around a closed laser gyroscope path defined by four mirrors. Two beams circulate in the clockwise direction and two in the counterclockwise direction. Of the two clockwise circulating beams, one is of left-hand circular polarization and the other of right-hand circular polarization as is also the case for the two counterclockwise circulating beams. In the preferred embodiment, the two beams of right-hand circular polarization are of higher frequency than those of left-hand circular polarization. A Faraday rotator structure provides the frequency difference or splitting between the beams of clockwise and counterclockwise rotation while the crystal rotator structure provides the frequency splitting between the beams of right-hand and left-hand polarization.
The relative frequency positions of the beams of four different frequencies are shown in the diagram of FIG. 2. To avoid the mode locking problem, the Faraday rotator provides sufficient frequency splitting between the beams of frequency f.sub.1 and f.sub.2 as well as between the beams of frequencies f.sub.3 and f.sub.4 at a zero rate of rotation and for all anticipated rates of rotation such that no mode locking can occur and the system is biased substantially outside the nonlinear region where mode locking occurs.
At rest, the frequency difference between the beams of frequency f.sub.1 and f.sub.2 is the same as that between the beams of frequencies f.sub.3 and f.sub.4. As the laser gyroscope system is rotated in a first direction, the beams of frequencies f.sub.1 and f.sub.2 will move together in frequency while those of f.sub.3 and f.sub.4 will move apart in frequency by the same amount. For rotation in the opposite direction, the beams of frequency f.sub.1 and f.sub.2 will move apart in frequency while those of f.sub.3 and f.sub.4 will move together by the same amount.
To produce an output signal having a frequency in proportion to the rate of rotation, a first two output frequency difference signals having frequencies .DELTA.f.sub.1 =f.sub.2 -f.sub.1 and .DELTA.f.sub.2 =f.sub.4 -f.sub.3 are formed. A final output signal .DELTA.f=.DELTA.f.sub.2 -.DELTA.f.sub.1 is then formed. To provide an indication of the total amount of rotation, two counters are provided, one of which is incremented by the .DELTA.f.sub.1 signal and the other by the .DELTA.f.sub.2 signal. The output of one counter is digitally subtracted from that of the other thus providing a digital signal indicative of the total amount of rotation of the system.
Although this system described in the Andringa patents has been found to function quite satisfactorily for numerous applications, in still further applications it has been found desirable to provide an output signal indicating either the amount of rotation or rate of rotation having a higher degree of precision than quantizing the .DELTA.f.sub.1 and .DELTA.f.sub.2 signals at one pulse per cycle of the signals can provide.